Quadriwave lateral shearing interferometric microscopy with wideband sensitivity enhancement for quantitative phase imaging in real time

Abstract

Real-time quantitative phase imaging has tremendous potential in investigating live biological specimens in vitro.Here we report on a wideband sensitivity-enhanced interferometric microscopy for quantitative phase imaging in real time by employing two quadriwave lateral shearing interferometers based on randomly encoded hybrid gratings with different lateral shears. Theoretical framework to analyze the measurement sensitivity is firstly proposed, from which the optimal lateral shear pair for sensitivity enhancement is also derived. To accelerate the phase retrieval algorithm for real-time visualization, we develop a fully vectorized path-independent differential leveling phase unwrapping algorithm ready for parallel computing, and the framerate for retrieving the phase from each pair of two 4 mega pixel interferograms is able to reach 47.85 frames per second. Experiment results demonstrate that the wideband sensitivity-enhanced interferometric microscopy is capable of eliminating all the periodical error caused by spectral leaking problem and reducing the temporal standard deviation to the half level compared with phase directly retrieved by the interferogram. Due to its high adaptability, the wideband sensitivity-enhanced interferometric microscopy is promising in retrofitting existing microscopes to quantitative phase microscopes with high measurement precision and real-time visualization.Quantitative phase microscopy (QPM), which provides additional phase information about the refractive index distribution of transparent specimens, has drawn much attention in both optical and biomedical research. The main advantage of quantitative phase microscopy (QPM) over conventional intensity imaging or fluorescence microscopy is that it is label-free and no dye or fluorescent protein is required to enhance the contrast of the microscopic image, which makes it possible to study live biological specimens in vitro [1][2][3][4][5] . Ptychography 5-8 is one of the emerging QPM techniques that can retrieve the phase and amplitude of the scattered wavefield when a coherent illumination beam passes through the specimen under test. The diffraction patterns of the specimen are captured by a CCD camera and the phase retrieval algorithm is carried out based on the extended ptychographic iterative engine (ePIE). Ptychographic microscope can obtain high contrast images free of halo artefact using low magnification microscopic objectives 9, 10 . However, both the acquisition for the tile scan to generate overlapping diffraction patterns and the computation of the ePIE algorithm takes a relatively long time, so it is currently not available to real-time applications. Different from ptychography, the transport-of-intensity equation (TIE) technique employs focus and defocus images to retrieve the phase variation of the wavefront. It can be applied to a conventional intensity microscope directly and the acquisition rate can be set up to 15Hz if an electrically tunable fluidic lens is employed to vary the equivalent focal length of the opt...

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